The invention relates to manhole/access cover and frame assemblies. The invention is particularly applicable to assemblies where both the cover and frame are formed from cast iron, but it is not limited to such assemblies.
It is frequently required that a manhole/access cover and frame assembly be rendered gas, air and water tight. This is normally required, for example, where the assembly is located within a domestic building area and covers, for example, a storm water or foul sewer chamber. In order to achieve the necessary seal it is conventional practice to provide the cover and frame with interengaging sealing surfaces which extend continuously around the periphery of the cover and between which surfaces a sealing compound, usually grease, may be spread to form an effective seal. In conventional assemblies of this type the interengaging sealing surfaces also provide the support for the cover within the surrounding frame so that, in theory, the cover is supported around the whole of its periphery. However, since the cover and frame are normally cast in iron, perfect accuracy in the shaping and finish of the sealing surfaces is difficult to achieve and, in practice, most frames and covers are slightly misshapen for one reason or another. Because of this misshaping there is a tendency for the cover to rock in the frame and such rocking will eventually break down the grease seal at the periphery of the frame, rendering it effective.
One method of overcoming this problem has been to ensure that the cover is firmly secured to the frame, for example by use of brass screws at the corners of a rectangular cover. The screws also have to be sealed where they enter the top of the cover and it will be appreciated that this solution is both expensive and inconvenient. An alternative solution has been to machine finish the engaging surfaces of the cover and frame to ensure a better fit than is given by the cover and frame as cast, but again this increases the cost of manufacture of the assembly.
The present invention sets out to provide a manhole/access cover and frame assembly which can provide a gas, air and water tight seal, without risk of rocking of the cover, and yet where the cover and frame may be used in the "as cast" condition.